(a) Field of the Invention
The present invention relates to a transmission system in an orthogonal frequency division multiplexing method. More particularly, the present invention relates to a device for compensating a distortion of a transmission signal, the distortion caused by a digital/analog converter, and a method thereof.
(b) Description of the Related Art
In an orthogonal frequency division multiplexing transmission system, digital data modulated in various modulation methods are robust against distortions caused by wireless multipaths since orthogonal frequencies are used to transmit the modulated digital data, and a bidirectional communication may be performed by using a simplified receiver. Particularly, in an orthogonal frequency division modulation system, there is a merit in that a function of an orthogonal frequency division modulation system may be realized in a domain that is efficient for calculation among frequency and time domains since the digital data are in the frequency and time domains due to a fast Fourier transform unit.
The orthogonal frequency division multiplexing wireless transmission system inverse-fast-Fourier transforms respective in-phase and quadrature-phase signals to convert them to digital signals by a digital/analog converter, and then wirelessly transmits the converted signals. A transmission signal converted to be a time domain digital signal by an inverse fast Fourier transform unit may be converted to the analog signal to be wirelessly transmitted. In this case, since the transmission signal is converted to the analog signal at a clock frequency which is the same as that of the digital signal, a signal distortion occurs by a sample/hold operation of the digital/analog converter at a radio frequency domain in a band. In order to prevent the signal distortion, the distortion is compensated by a digital filter at a time domain, or a digital signal having a high sample rate is converted to an analog signal after interpolation is performed by using a multiple of a sample frequency of a digital input signal, which will be described with FIG. 1.
FIG. 1 shows a schematic diagram of a transmitter in an orthogonal frequency division multiplexing transmission system according to the prior art.
As shown in FIG. 1, the orthogonal frequency division multiplexing transmission system according to the prior art includes a digital signal processor 110 including a digital modulator 101, an inverse fast Fourier transform (IFFT) unit 102, a parallel/serial data converter 103, a windowing or filtering unit 104, an inverse sync compensation filter 105, and a digital/analog converter 106, and an analog signal processor 120 including an in-phase and quadrature-phase (IQ) analog modulator 107, a radio frequency (RF) transmitter 108, and an antenna 109.
Referring to FIG. 1, a transmission digital signal is modulated by the digital modulator 101 in digital modulation methods including an M-phase shift keying (M-PSK) and an M-quadrature amplitude modulation (M-QAM), and is output as in-phase (I) and quadrature-phase (Q) signals.
The output IQ signals are respectively transformed from a frequency domain signal to a time domain signal by the inverse Fourier transform unit 102. The transformed signal is reconverted to the time domain signal by the parallel/serial data converter 103, and, if necessary, an interpolation operation may be performed by using the windowing or filtering unit 104. Since the above process requires various operations and a radio frequency clock signal, it may not be used in a potable terminal in which power consumption is very important.
Accordingly, the digital filter is not used in a transmitter of a portable terminal of the orthogonal frequency division multiplexing method. Since the digital signal input to the digital/analog converter 106 has the same sample rate as that of the output signal of the parallel/serial data converter 103, a sync function having a Null at a frequency corresponding to the signal sample rate causes a distortion at a frequency domain.
To compensate the distortion caused by the digital/analog converter 106, the input signal is compensated at the time domain by the inverse sync compensation filter 105 as a digital filter. However, since a considerable number of operations is required to form a filter in the above method, it is not easily used in the portable terminal.
Subsequently, the output IQ signals of the digital/analog converter 106 is converted to an intermediate frequency (IF) signal by the IQ analog modulator 107, the converted IF signal is converted to an RF signal and is transmitted to the antenna 109 by the RF transmitter 108, and the RF signal is wirelessly transmitted through the antenna 109.
As described above, in the conventional compensation method at the time domain, a filtering method is used to compensate a signal at the time domain since data at the frequency domain are difficult to be obtained in a conventional digital communication or in a code division multiple access (CDMA) communication, but a considerable number of operations is performed. In addition, in a method using the interpolation, an attenuation is generated relatively less in a band of an original signal to be transmitted since a zero point of the sync function is generated at a radio frequency by increasing a sample frequency of the digital/analog converter. However, there is a problem in that the number of operations is increased since the digital filter is used when the interpolation is performed, and a digital/analog converter operating at the radio frequency is used.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.